1. Field of the Invention
The present invention generally relates to the electronics industry, and more specifically to radio transceiver systems. The present invention more specifically relates to a multiband bidirectional coupler.
2. Discussion of the Related Art
A coupler is generally used to sample part of the power present on a so-called main or primary transmission line, to direct it towards another so-called coupled or secondary line, located nearby. Couplers can be divided in two categories according to whether they are formed of discrete passive components (lumped-element couplers) or of conductive lines close enough to one another to be coupled (distributed couplers). The present invention relates to the second category of couplers. The ports of the main line are generally designated as IN (input) and OUT (output). Those of the coupled line are generally designated as CPLD (coupled) and ISO (isolated).
In many applications, part of the power transmitted over a line needs to be sampled, for example, to control the power of an amplifier in a transmission system, to control the linearity of a transmitter amplifier according to the losses linked to the reflection of an antenna, to dynamically match an antenna, etc.
The main parameters of a coupler are:
insertion losses, which represent the transmission loss between ports IN and OUT of the main line (insertion losses are defined while the two other ports CPLD and ISO of the coupler are loaded with a 50-ohm impedance);
the coupling, which represents the transmission loss between ports IN and CPLD (the coupling is then defined while the two ports OUT and ISO are loaded with a 50-ohm impedance);
the isolation, which represents the transmission loss between ports IN and ISO (the isolation is defined while the two other ports OUT and CPLD are loaded with a 50-ohm impedance); and
the directivity which represents the transmission loss difference between ports ISO and CPLD, from port IN.
Theoretically, an ideal coupler exhibits an infinite directivity, that is, no power is present on the port of its secondary line located in front of the output port of its main line when a signal flows from the input port to the output port of this main line. In practice, a coupler is said to be directional when its directivity is sufficient (typically greater than 20 dBm) for the powers recovered on the accesses of its secondary line to enable to determine the power flow direction in the main line. When the two ports of the secondary line of the coupler are used to simultaneously have the power information, the coupler is said to be bidirectional.
Increasingly, radio transceiver devices are capable of operating in several frequency bands. Such is for example the case for mobile telephony, where cell phones have evolved from bi-band to tri-band, then to quadband.
The transceiver chain then comprises as many paths as the device is able to process frequency bands, both in transmit and in receive mode. Each path is associated with a coupler sized according to the frequency band to be processed. In particular, the lengths of the main and secondary lines depend on this frequency band.
This need for a different sizing of the couplers complicates the manufacturing. Further, with couplers of different lengths, there are directivity variations from one coupler to the other, which is not desirable.
In a coupler, if the two ports of its secondary line and the output port of its main line are perfectly matched, no stray reflection occurs. Such a perfect matching can unfortunately not be obtained in practice. In particular, the port on which the power portion is sampled by coupling is seldom ideally matched. As a result, stray reflections generate errors on the recovered information.
A mismatch of the port of the secondary line of the coupler from which the information is sampled may have different origins. Most often, the coupler is placed on an insulating substrate (for example, of printed circuit type) to be associated with other circuits. It is then not possible to ensure a perfect matching (typically, 50 ohms) of the measurement port. Further, if the couplers have different sizes, this matching risks to vary from one coupler to the other.